Olefin hydrocarbons are useful for the production of a number of petrochemical products, such as polymers, motor fuel blending additives, and other products. Short chain saturated hydrocarbons having from 2 to 5 carbon atoms per molecule are often subjected to dehydrogenation to form the corresponding olefin. The olefins, in turn, may be used in the alkylation of isoparaffins, in the etherification of alcohols to make motor fuel blending additives, or as monomers used to produce various polymer materials.
One particularly useful olefin is propylene, which is produced by dehydrogenation of propane. Propylene is the world's second largest petrochemical commodity and is used in the production of polypropylene, acrylonitrile, acrylic acid, acrolein, propylene oxide and glycols, plasticizer oxo alcohols, cumene, isopropyl alcohol and acetone. The growth in propylene production is primarily driven by the industry demand for polypropylene, which is used in such everyday products as packaging materials and outdoor clothing.
Propylene is primarily produced from the dehydrogenation of propane. A conventional propane dehydrogenation process involves the following steps: dehydrogenation of propane to propylene in a reactor, compression of the reactor effluent, and recovery and purification of the propylene product. FIG. 1 shows the steps in a conventional dehydrogenation process. In the dehydrogenation step (1), the conversion of propane to propylene is typically carried out over a catalyst. The effluent from the dehydrogenation unit is compressed in a compressor (2) to a sufficiently high pressure, typically 150 psig or greater, to recover unreacted propane and propylene from lighter components in a recovery section.
In the recovery step (3), the compressed reactor effluent is successively chilled with refrigeration to maximize the recovery of propane and propylene for further purification. The offgas from this process mainly consists of hydrogen, methane, ethane and ethylene. The hydrocarbons from the recovery step (3) are then subjected to distillation in the purification step (4). In a first distillation column, a deethanizer, ethane and lighter gases are recovered as overhead offgas, and propane and propylene are recovered in the deethanizer bottoms stream. In a second distillation column, generally referred to as a product splitter, propylene product is recovered as overhead and propane from the bottoms is recycled back to the dehydrogenation step. A purge stream of fresh propane and recycled propane are distilled to remove heavier components from the process. During purification, the product splitter is typically reboiled by using an external heat source (e.g. heat pump) whereby the overhead vapor is compressed and used as the reboiling medium.
One common process for production of propylene by dehydrogenation of propane is known as the CATOFIN process. In the CATOFIN process, propane is converted to propylene by feeding propane to a dehydrogenation reactor containing a fixed bed Chromium-Alumina catalyst. There are typically multiple dehydrogenation reactors operating in parallel to allow catalyst regeneration to occur in some reactors while others are in operation. The dehydrogenation reactors are typically maintained at about 600-650° C.
The effluent from the dehydrogenation reactors is cooled and compressed using a steam driven product compressor. The compressed product is sent to a recovery section where inert gases, hydrogen and light hydrocarbons are removed from the compressed reactor effluent. The propylene rich gas from the recovery unit is then sent to the product purification section where propylene is separated from propane as described above.
The purification step of a conventional propane dehydrogenation process is shown in FIG. 2. The product splitter (110) in the conventional process is fed the heavy end from a deethanizer which contains C3+ compounds through feed line (100). This feed is distilled in the product splitter such that the propylene product is recovered in the overhead stream (102) and the majority of the remaining compounds, including unreacted propane, exit in the bottoms stream (128). This conventional product splitter is operated at pressures of about 80-100 psig and temperatures of 40-60° F.
The overhead propylene vapor stream (102) is combined with the overhead (105) from separator (150) and sent to heat pump (130) through line (106). The heat pump is driven by steam turbine (131) using high pressure steam provided through line (133). The exhaust steam is discharged through line (122) to condenser (160), where it is cooled and discharged from the plant.
The overhead vapor stream (102) is compressed in heat pump (130) and flows through discharge line (108) to provide heat to product splitter reboiler (120). The warmed propylene is split, with a portion flowing back to the product splitter through line (114), and the remainder flowing through line (112) to product separator (150). The overhead (105) from the separator (150) is combined with the overhead propylene stream (102) from the product splitter and fed to the heat pump (130) as described above. The propylene product (118) from the bottoms of the separator is sent to other units for further processing.
The product compression machine is driven by steam turbine (141) which is fed high pressure steam through line (143). The product compressor is fed the product from the dehydrogenation reactor (not shown) through line (127) for compression. The compressed dehydrogenation product is fed through line (126) for separation in a deethanizer. In conventional plants, the exhaust steam from the steam turbine (141) is discharged through line (124) to condenser (170), where it is cooled and discharged from the plant.
The bottoms of the conventional low pressure product splitter comprises mainly propane. The bottoms are discharged through line (128) and split, with a portion of the bottoms recycled through line (104) to reboiler (120), where it is heated and sent back to the product splitter (110). The remainder of the bottoms are discharged through line (116) and sent back to the dehydrogenation reactors.
This conventional dehydrogenation process has some inherent limitations. One primary limitation is the amount of input energy required to produce the propylene product. Currently, the total energy consumption for the conventional dehydrogenation process, for example, is about 100 kcal/kg of propylene product. As such, there exists an ongoing and unmet need in the industry for a less expensive and more efficient method for dehydrogenation of propane.